(318h) Comparative Analysis of Surface Configurations of CO Adsorbed on hcp and fcc Cobalt for the Fischer-Tropsch Synthesis

The first step in
Fischer-Tropsch (FT) synthesis over Co catalysts is carbon monoxide (CO)
adsorption, which by and
large out-competes dissociative hydrogen adsorption. Along with the fact that
CO dissociation is not easy on the basal planes of Co catalysts, this fact implies
that associatively adsorbed CO will likely have time to equilibrate with the
imposed CO atmosphere and take on configurations relevant to the subsequent
steps of the reaction. We present here a density functional theory (DFT) based
investigation of these relevant configurations, taking into account the lateral
interactions of CO through the construction of highly-predictive lattice gas
models. Since both the face centered cubic (fcc) and hexagonal close packed
(hcp) phases of Co have been shown to exhibit different FT activity, we compare
these LG models and configurations as a function of the fcc and hcp phases; the
primary difference being a large change in site-to-site distance (i.e.
different lattice strains). 

By utilizing the Alloy
Theoretic Automated Toolkit to generate a large variety of extended Co(111) and
Co(0001) surfaces with different coverages and configurations of CO, we
construct a large library of CO/Co(111) and CO/Co(0001) structures. A LG model
is fit to these data and subsequently optimized to produce the most predictive
model possible for each system. The results reveal a non-trivial change in the
first nearest neighbor (NN) interaction energy, reflected in a much more
favorable full-coverage in the hcp surface (see Figure 1). However, we are able
to show that the ground states of these two systems are largely unchanged
despite this difference. Our results facilitate experimentally relevant
comparisons of the two systems and indicate what kind of surface structures one
should expect to be present in the early stages of FT synthesis given an hcp
vs. fcc Co-based catalyst.

Figure 1. Surface energies and associated
LG predicted energies for the hcp and fcc Co systems.